When a subject is photographed with a digital camera, there has hitherto been a case where the brightness of a subject exceeds a dynamic range of an image sensor when photometric measurement is performed with brightness of the subject being unknown as in the initial start-up stage of a digital camera, switching of a photography mode, e.g., from a review mode to a photography mode, and the like. In such a case, photographing is performed a plurality of times while exposure conditions are being altered, in order to perform photographing operation under appropriate exposure conditions.
In a case where photographing is performed with strobe light being fired, there arises a case where, when the amount of light of the strobe is adjusted by use of firing of preliminary light (strobe light control), photometric measurement over a long range cannot be carried out by a single firing of preliminary light because of limitations on the dynamic range of an image sensor. Even in this case, preliminary flash light is fired a plurality of times while firing conditions are changed. Moreover, in the case of high photographic sensitivity or a high flash guide number, a strobe range becomes longer. When the dynamic range of the image sensor is narrow, there arises a case where photometric measurement cannot be carried out by a single firing of preliminary flash light. Even in this case, preliminary flash light must be fired a plurality of times with the amount of light being changed.
FIG. 8 shows an operation timing chart which is used to set the amount of light to be fired by means of the main flash by firing preliminary flash light a plurality of times. The chart is illustrated while being generally divided into a monitoring period before photography, an evaluation period for controlling the amount of strobe light, and a still image photographing period which is the main photography operation. During the monitoring period, electric charges accumulated in an image sensor such as a CCD are transferred and read as an image signal, and the image signal is subjected to image processing such as compression. The processed image signal is displayed on a display device such as an LCD or the like. A vertical synch signal VD is a signal output every screen of the CCD, and is generated by a timing generator. In synchronism with the vertical synch signal VD, a read pulse SG is supplied to the image sensor, thereby determining read timing. After unwanted electric charges accumulated in the CCD are swept out by means of a sweep pulse signal Sub, exposure is initiated. The accumulated electric charges are sequentially read from the CCD by means of a vertical transfer clock signal VT. During an amount-of-strobe-light detection and evaluation period, a strobe built in or externally attached to the digital camera is driven, to thus effect a first firing of preliminary flash light (preliminary flash) (preliminary flash light 1 in the drawing). The electric charges accumulated in the CCD in the interim are read as an image signal. The thus-read image signal is provided not for display purpose but for photometric measurement purpose. A controller or a microprocessor of the digital camera determines the amount of a second firing of preliminary flash light, on the basis of the result of photometric measurement. In the drawing, the amount of the second firing of preliminary flash light (preliminary flash light 2) is made greater than the amount of the first firing of preliminary flash light. Read timing is determined by the read pulse signal SG, and unwanted electric charges are swept out by the sweep pulse signal Sub, and exposure is initiated. The electric charges accumulated in the CCD during the period of the second firing of preliminary flash light are read as an image signal by use of a vertical transfer clock signal VT. The thus-read image signal is provided not for display purpose but for photometric measurement purpose. On the basis of the first and second firings of preliminary flash light, the controller determines the amount of flash light fired as the main flash, thereby photographing a still image.
Japanese Patent Laid-Open Publication No. 2003-114465 provided below mentions that, when the amount of light based on the first firing of preliminary light (preliminary flash light) is a predetermined value or less, second preliminary flash light is fired while the intensity of flash light is made greater, thereby setting a guide number.
Japanese Patent Laid-Open Publication No. 2004-272071 mentions that intermittent preliminary flash light is fired by means of a PWM trigger pulse signal.
Japanese Patent Laid-Open Publication No. 2005-115161 mentions that the number of times preliminarily flash light is fired and the amount of preliminary flash light are set on the basis of the maximum guide number and the minimum guide number of a strobe as well as on the basis of a dynamic range of a sensor in a dimmer circuit.
However, a configuration for effecting photography a plurality of times or firing preliminary light a plurality of times presents a problem of photoelectric measurement involving consumption of time. Moreover, when preliminary light is fired a plurality of times, many electric charges accumulated in a capacitor provided in a strobe firing control circuit are consumed, thereby posing a problem of a decrease in the guide number during main photography.